Device and method for controlling a belt-type continuously variable transmission for a vehicle

ABSTRACT

A vehicle belt-type continuously variable transmission expands the operating range in which belt slip control is permitted with maintained estimated accuracy of a belt slip condition, to reduce drive energy consumption owing to a reduction in belt friction. It includes a primary pulley and a secondary pulley around which a belt is wound to control gear ratio by controlling primary and secondary hydraulic pressures, a belt slip control means for controlling secondary hydraulic pressure according to a phase difference between oscillation components due to oscillation included in an actual gear ratio and in an actual secondary hydraulic pressure, and a transmission speed limiting means for determining whether or not to limit vehicle acceleration according to a predetermined acceleration limit permitting condition and limiting a transmission speed to less than a predetermined value when determining to limit the vehicle acceleration.

TECHNICAL FIELD

The present invention relates to a device and a method for controlling avehicle belt type continuously variable transmission to perform a beltslip control in which a belt wound around pulleys is slipped at apredetermined slip rate.

BACKGROUND ART

A known belt type continuously variable transmission controller isconfigured to perform a belt slip control in which an actual secondaryhydraulic pressure is reduced from one during a normal control to slip abelt wound around pulleys at a predetermined slip rate by the followingsteps:

-   -   (a) superimposing a predetermined sine wave on a command        secondary hydraulic pressure or oscillating the command        secondary hydraulic pressure, and    -   (b) performing the belt slip control by controlling the actual        secondary hydraulic pressure on the basis of a multiplier of an        oscillation component due to the oscillation included due to the        oscillation in the actual secondary hydraulic pressure and an        oscillation. component included in an actual gear ratio.

This eliminates the necessity for directly detecting the belt slip rateand thereby facilitates the belt slip control (see Patent Document 1,for example).

Prior Art Document Patent Document

Patent Document 1: WO 2009/007450 A2 (PCT/EP2008/059092)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, the known belt type continuously variable transmissioncontroller cannot extract the oscillation component due to theoscillation from the basic component of a fluctuation characteristic ofthe actual gear ratio which corresponds to a driving condition when atransmission speed as a change rate of the gear ratio is high. Becauseof this, by performing belt slip control using, as a belt slip conditionestimated value, a multiplier of the oscillation components included inthe actual gear ratio and in the actual secondary hydraulic pressure tohave the value coincide with a predetermined value, the belt may greatlyslip depending on a magnitude of the input torque to the belt typecontinuously variable transmission controller due to an insufficientcontrol accuracy caused by an error in the estimated belt slipcondition.

In view of the above problem, the present invention aims to provide avehicle belt type continuously variable transmission control device andmethod which can improve the reducing effects of drive energyconsumption owing to a decrease in belt friction by expanding anoperation range in which the belt slip control is permitted, with theaccuracy of an estimated belt slip condition maintained.

Means to Solve the Problem

In view of achieving the above object. a belt type continuously variabletransmission according to the present invention comprises a primarypulley and a secondary pulley around which a belt is wound, to control agear ratio by controlling a primary hydraulic pressure and a secondaryhydraulic pressure. A control device therefor comprises a belt slipcontrol means, a limit determining means, and a transmission speedlimiting means. The belt slip control means is configured to oscillatethe secondary hydraulic pressure and extract an oscillation componentdue to the oscillation included in an actual gear ratio from a basiccomponent of the actual gear ratio when a transmission speed is lessthan a predetermined value, so as to control the secondary hydraulicpressure on the basis of a phase difference between the oscillationcomponent due to the oscillation included in the actual gear ratio andan oscillation component due to the oscillation included in an actualsecondary hydraulic pressure, the transmission speed being a changespeed of the gear ratio. The limit determining means is configured todetermine whether to limit acceleration of the vehicle on the basis of apredetermined acceleration limit permitting condition. The transmissionspeed limiting means is configured to limit the transmission speed toless than the predetermined value when the limit determining meansdetermines to limit the acceleration of the vehicle.

Effects of the Invention

The belt slip control means is configured to permit the belt slipcontrol when the transmission speed as a change speed of the gear ratiois less than the predetermined value. Thereby, with a high estimatedaccuracy of a belt slip condition, driving energy consumption can bereduced by a decrease in belt friction while with a low estimatedaccuracy of a belt slip condition, a large belt slippage during the beltslip control can be prevented.

In addition, when the limit determining means decides that theacceleration of the vehicle can be limited, the transmission speedlimiting means limits the transmission speed to less than thepredetermined value to expand an operation range in which the belt slipcontrol is permitted. That is, the operation range can be expanded by abelt slip control permitted range under the acceleration limitpermitting condition from that under the transmission speed conditionwhich depends on the change speed of gear ratio which occurs with adriving condition. Accordingly, the estimated accuracy of the belt slipcondition can be maintained by limiting the transmission speed to lessthan the predetermined value in the added operation range.

Thus, it is made possible to effectively reduce the driving energyconsumption owing to a decrease in belt friction by expanding theoperation range in which the belt slip control is permitted with theestimated accuracy of the belt slip condition maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the entire system of a drive system and a control system ofa vehicle incorporating a belt type continuously variable transmissionapplied with a control device and method according to a firstembodiment.

FIG. 2 is a perspective view of the belt type continuously variabletransmission mechanism applied with the control device and methodaccording to the first embodiment.

FIG. 3 is a perspective view of a part of a belt of the belt typecontinuously variable transmission mechanism applied with the controldevice and method according to the first embodiment.

FIG. 4 is a control block diagram of the line pressure control andsecondary hydraulic pressure control (normal control/belt slip control)executed by a CVT control unit 8 according to the first embodiment.

FIG. 5 is a flowchart for a basic switching process between the normalcontrol and the belt slip control (=BSC) over the secondary hydraulicpressure executed by the CVT control unit 8 according to the firstembodiment.

FIG. 6 is a flowchart for the entire belt slip control process executedby the CVT control unit 8 according to the first embodiment.

FIG. 7 is a flowchart for the torque limit process of the belt slipcontrol process executed by the CVT control unit 8 according to thefirst embodiment.

FIG. 8 is a flowchart for the secondary hydraulic pressure oscillationand correction process of the belt slip control process executed by theCVT control unit 8 according to the first embodiment.

FIG. 9 is a flowchart for a returning process from the belt slip controlto the normal control executed by the CVT control unit 8 according tothe first embodiment.

FIG. 10 is a flowchart for the torque limit process of the returningprocess to the normal control executed by the CVT control unit 8according to the first embodiment.

FIG. 11 is a flowchart for limiting process for the transmission speedof a gear ratio to limit a target primary rotary rate in the returningprocess to the normal control executed by the CVT control unit 8according to the first embodiment.

FIG. 12 is a flowchart for the entire BSC permission determining processexecuted by the CVT control unit 8 according to the first embodiment.

FIG. 13 shows a threshold characteristic as a threshold 1 for throttleopening used when an ECO switch is turned on in the BSC permissiondetermining process executed by the CVT control unit 8 according to thefirst embodiment.

FIG. 14 shows a threshold characteristic as a threshold 2 for throttleopening used when an ECO switch is turned off in the BSC permissiondetermining process executed by the CVT control unit 8 according to thefirst embodiment.

FIG. 15 is a timing chart of an actual gear ratio characteristic and atarget gear ratio characteristic when transmission is changed at a smalltransmission change rate during the belt slip control.

FIG. 16 is a timing chart of an actual gear ratio characteristic and atarget gear ratio characteristic when transmission is changed at a largetransmission change rate during the belt slip control.

FIG. 17 is a timing chart for the respective characteristics of limitdetermination, target gear ratio, BSC operation flag (for comparison),limited target gear ratio, and limited BSC operation flag when thetarget gear ratio is fluctuated by a driver's slight change to theaccelerator opening.

FIG. 18 is a graph showing the effects of an expanded BSC operationrange according to the first embodiment in comparison with the targettransmission change rate not actively limited.

FIG. 19 is a timing chart of the respective characteristics of BSCoperation flag, SEC pressure F/B inhibition flag, accelerator opening,vehicle speed, engine torque, Ratio, SEC hydraulic pressure, SEC_SOLcurrent correction amount, and phase difference between SEC pressureoscillation and Ratio oscillation in a traveling scene during a controlshift from the normal control, belt slip control, returning control tothe normal control.

FIG. 20 is a timing chart of the characteristics of driver requesttorque, torque limit amount, torque capacity, and actual torque toillustrate the torque limit control based on torque delay used in thereturning control from the belt slip control to the normal control.

EMBODIMENTS OF DESCRIPTION

Hereinafter, the best mode to carry out the control device and methodfor a belt type continuously variable transmission will be describedusing a first embodiment with reference to the accompanying drawings.

First Embodiment

First, the structure of the device is described. FIG. 1 shows the entiresystem of a drive system and a control system of a vehicle incorporatinga belt type continuously variable transmission applied with a controldevice and method according to the first embodiment. FIG. 2 is aperspective view of the belt type continuously variable transmissionmechanism applied with the control device and method according to thefirst embodiment. FIG. 3 is a perspective view of a part of a belt of abelt type continuously variable transmission mechanism applied with thecontrol device and method according to the first embodiment. In thefollowing the system structures are described with reference to FIGS. 1to 3.

In FIG. 1 the drive system of a vehicle incorporating a belt typecontinuously variable transmission comprises an engine 1, a torqueconverter 2, a forward/backward drive switch mechanism 3, a belt typecontinuously variable transmission mechanism 4, a final reductionmechanism 5 and drive wheels 6, 6.

The output torque of the engine 1 is controllable by an engine controlsignal supplied from the exterior in addition to by a driver'sacceleration operation. The engine 1 includes an output torque controlactuator 10 to control the output torque by a throttle valveopening/closing operation, a fuel cut operation and else. The changespeed (=change rate) of the input torque to the belt type continuouslyvariable transmission 4 is controlled by the output torque control ofthe engine 1.

The torque converter 2 is a startup element with a torque increasingfunction and includes a lockup clutch 20 to be able to directly connectan engine output shaft 11 (=torque converter input shaft) and a torqueconverter output shaft 21. The torque converter 2 is comprised of aturbine runner 23 connected with the engine output shaft 11 via aconverter housing 22, an impeller pump 24 connected with the torqueconverter output shaft 21, and a stator 26 provided via a one-way clutch25.

The forward/backward drive switch mechanism 3 is to switch a rotarydirection input to the belt type continuously variable transmissionmechanism 4 between a normal rotary direction during forward travelingand a reverse rotary direction during backward traveling. Theforward/backward switch mechanism 3 includes a double pinion planetarygear 30, a forward clutch 31, and a backward brake 32. A sun gear of thedouble pinion planetary gear 30 is connected with the torque converteroutput shaft 21 and a carrier thereof is connected with a transmissioninput shaft 40. The forward clutch 31 is fastened during a backwardtraveling to fix a ring gear of the double pinion planetary gear 30 tothe case.

The belt type continuously variable transmission mechanism 4 has acontinuously variable transmission function to steplessly vary the gearratio by changing a belt contact radius. The gear ratio is a ratio ofthe input rotation rate of the transmission input shaft 40 and theoutput rotation rate of the transmission output shaft 41. The belt typecontinuously variable transmission mechanism 4 includes a primary pulley42, a secondary pulley 43, and a belt 44. The primary pulley 42 is madeup of a fixed pulley 42 a and a slide pulley 42 b. The slide pulley 42 bis slid by primary hydraulic pressure introduced into a primaryhydraulic pressure chamber 45. The secondary pulley 43 is made up of afixed pulley 43 a and a slide pulley 43 b. The slide pulley 43 b is slidby primary hydraulic pressure introduced into a secondary hydraulicpressure chamber 46. The belt 44 as shown in FIG. 2 is wrapped aroundV-form sheave faces 42 c, 42 d of the primary pulley 42 and V-formsheave faces 43 c, 43 d of the secondary pulley 43. In FIG. 3 the belt44 is formed of two laminated rings 44 a, 44 a of which a large numberof rings are layered from inside to outside as well as a large number ofelements 44 b of press-cut plates placed between the two laminated rings44 a, 44 a and connected with each other in a ring-form. The elements 44b each includes, at both sides, flank faces 44 c, 44 c to contact withthe sheave faces 42 c, 42 d of the primary pulley 42 and the sheavefaces 43 c, 43 d of the secondary pulley 43.

The final reduction mechanism 5 decelerates the transmission outputrotation from the transmission output shaft 41 of the belt typecontinuously variable transmission mechanism 4 and provides adifferential function thereto to transmit it to the right and left drivewheels 6, 6. The final reduction mechanism 5 is interposed among thetransmission output shaft 41, an idler shaft 50, right and left driveshafts 51, 51, and includes a first gear 52, a second gear 53, a thirdgear 54, and a fourth gear 55 with a deceleration function and a geardifferential gear 56 with a differential function.

The control system of the belt type continuously variable transmissioncomprises a transmission hydraulic pressure control unit 7 and a CVTcontrol unit 8, as shown in FIG. 1.

The transmission hydraulic pressure control unit 7 is a hydraulicpressure control unit to produce primary hydraulic pressure introducedinto the primary hydraulic pressure chamber 45 and secondary hydraulicpressure introduced into the secondary hydraulic pressure chamber 46.The transmission hydraulic pressure control unit 7 comprises an oil pump70, a regulator valve 71, a line pressure solenoid 72, a transmissioncontrol valve 73, a decompression valve 74, a secondary hydraulicpressure solenoid 75, a servo link 76 a transmission command valve 77,and a step motor 78.

The regulator valve 71 uses discharged pressure from the oil pump 70 asa pressure source to adjust line pressure PL. The regulator valve 71includes the line pressure solenoid 72 to adjust the pressure of oilfrom the oil pump 70 to a predetermined line pressure PL in response toa command from the CVT control unit 8.

The transmission control valve 73 uses the line pressure PL produced bythe regulator valve 71 as a pressure source to adjust the primaryhydraulic pressure introduced into the primary hydraulic pressurechamber 45. A spool 73 a of the transmission control valve 73 isconnected with the servo link 76 constituting a mechanical feedbackmechanism and the transmission command valve 77 connected with one endof the servo link 76 is driven by the step motor 78 so that thetransmission control valve receives feedback of a slide position (actualpulley ratio) from the slide pulley 42 b of the primary pulley 42connected with the other end of the servo link 76. That is, duringtransmission, when the step motor 78 is driven in response to a commandfrom the CVT control unit 8, the spool 73 a of the transmission controlvalve 73 is changed in position to supply/discharge the line pressure PLto/from the primary hydraulic pressure chamber 45 to adjust the primaryhydraulic pressure to acquire a target gear ratio commanded at the driveposition of the step motor 78. Upon completion of the transmission, thespool 73 a is held at a closed position in response to a displacement ofthe servo link 76.

The decompression valve 74 uses the line pressure PL produced by theregulator valve 71 as a pressure source to adjust the secondaryhydraulic pressure introduced into the secondary hydraulic pressurechamber 46 by decompression. The decompression valve 74 comprises thesecondary hydraulic pressure solenoid 75 to decompress the line pressurePL to a command secondary hydraulic pressure in accordance with acommand from the CVT control unit 8.

The CVT control unit 8 is configured to perform various control such asa gear ratio control to output to the step motor 78 a control command toacquire a target gear ratio in accordance with vehicle speed, throttleopening condition and else, a line pressure control to output to theline pressure solenoid 72 a control command to acquire a target linepressure in accordance with the throttle opening condition or else, asecondary hydraulic pressure control to output to the secondaryhydraulic pressure solenoid 75 a control command to acquire a targetsecondary pulley thrust in accordance with transmission input torque orelse, a forward and backward switch control to control the fastening andrelease of the forward clutch 31 and backward brake 32, and a lockupcontrol to control fastening and release of the lockup clutch 20. TheCVT control unit 8 receives various sensor information and switchinformation from a primary rotation sensor 80, a secondary rotationsensor 81 a secondary hydraulic pressure sensor 82, an oil temperaturesensor 83, an inhibitor switch 84, a brake switch 85, an acceleratoropening sensor 86, and other sensors and switches 87. Further, itreceives torque information from an engine control unit 88 and outputs atorque request to the engine control unit 88. In addition, it receivesswitch information from an ECO switch 89 (switch) to allow a driver toselect a normal drive mode or an economical drive mode.

FIG. 4 is a control block diagram of the line pressure control andsecondary hydraulic pressure control (normal control/belt slip control)executed by the CVT control unit 8 according to the first embodiment.

The hydraulic pressure control system of the CVT control unit 8 in thefirst embodiment comprises a basic hydraulic pressure calculator 90, aline pressure controller 91, a secondary hydraulic pressure controller92, a sine wave oscillation controller 93, and a secondary hydraulicpressure corrector 94, as shown in FIG. 4.

The basic hydraulic pressure calculator 90 includes an input torquecalculator 90 a to calculate transmission input torque on the basis ofthe torque information (engine rotary rate, fuel injection time and thelike) from the engine control unit 88 (FIG. 1), a basic secondary thrustcalculator 90 b to calculate a basic secondary thrust (belt clamp forcenecessary for the secondary pulley 43) from the transmission inputtorque obtained by the input torque calculator 90 a, a transmissionrequired thrust difference calculator 90 c to calculate a thrustdifference required for transmission (a difference in belt clamp forcebetween the primary and secondary pulleys 42, 43), a corrector 90 d tocorrect the calculated basic secondary thrust on the basis of therequired thrust difference for transmission, and a secondary hydraulicpressure converter 90 e to covert the corrected secondary thrust to atarget secondary hydraulic pressure. It further includes a basic primarythrust calculator 90 f to calculate a basic primary thrust (belt clampforce required for the primary pulley 42) from the transmission inputtorque calculated by the input torque calculator 90 a, a corrector 90 gto correct the calculated basic primary thrust on the basis of therequired thrust difference for transmission calculated by thetransmission required thrust difference calculator 90 c, and a primaryhydraulic pressure converter 90 h to convert the corrected primarythrust to a target primary hydraulic pressure.

The line pressure controller 91 includes a target line pressuredeterminer 91 a to compare the target primary hydraulic pressure outputfrom the primary hydraulic pressure converter 90 h with the commandsecondary hydraulic pressure output from the secondary hydraulicpressure controller 92, and set the target line pressure to the targetprimary hydraulic pressure when the target primary hydraulicpressure≧the command secondary hydraulic pressure and set the targetline pressure to the secondary hydraulic pressure when the targetprimary hydraulic pressure<the command secondary hydraulic pressure, anda hydraulic pressure-current converter 91 b to convert the target linepressure determined by the target line pressure determiner 91 a to acurrent value applied to the solenoid and output a command current valueconverted to the line pressure solenoid 72 of the regulator valve 71.

In the normal control the secondary hydraulic pressure controller 92performs the feedback control using the actual secondary hydraulicpressure detected by the secondary hydraulic pressure sensor 82 toacquire a command secondary hydraulic pressure while in the belt slipcontrol it performs open control without using the actual secondaryhydraulic pressure to acquire the command secondary hydraulic pressure.It includes a low pass filter 92 a through which the target secondaryhydraulic pressure from the secondary hydraulic pressure converter 90 eis filtered, a deviation calculator 92 b to calculate a deviationbetween the actual secondary hydraulic pressure and the target secondaryhydraulic pressure, a zero deviation setter 92 c to set the deviation tozero, a deviation switch 92 d to selectively switch between thecalculated deviation and zero deviation, and an integrated gaindeterminer 92 e to determine an integrated gain from oil temperature.Further, it includes a multiplier 92 f to multiply the integrated gainfrom the integrated gain determiner 92 e and the deviation from thedeviation switch 92 d, an integrator 92 g to integrate an FB integratedcontrol amount from the multiplier 92 f, an adder 92 h to add theintegrated FB integrated control amount to the target secondaryhydraulic pressure from the secondary hydraulic pressure converter 90 e,and a limiter 92 i to set upper and lower limits to the added value toobtain the command secondary hydraulic pressure (referred to basicsecondary hydraulic pressure in the belt slip control). Further, itincludes an oscillation adder 92 j to add a sine wave oscillationcommand to the basic secondary hydraulic pressure in the belt slipcontrol, a hydraulic pressure corrector 92 k to correct the oscillatedbasic secondary hydraulic pressure by a secondary hydraulic pressurecorrection amount to the command secondary hydraulic pressure, and ahydraulic pressure-current converter 92 m to convert the commandsecondary hydraulic pressure into a current value applied to thesolenoid to output a command current value converted to the secondaryhydraulic pressure solenoid 75. Note that the deviation switch 92 d isconfigured to select the calculated deviation when a BSC operation flagis 0 (during the normal control) and select the zero deviation when theBSC operation flag is 1 (during the belt slip control).

The sine wave oscillation controller 93 oscillates the secondaryhydraulic pressure by applying sine wave hydraulic pressure oscillationto the command secondary hydraulic pressure during the belt slipcontrol. It includes a sine wave oscillator 93 a to decide anoscillation frequency and an oscillation amplitude suitable foracquiring a phase difference between the oscillation component due tothe oscillation included in the actual secondary hydraulic pressure andthat included in the actual gear ratio and apply the sine wave hydraulicpressure oscillation in accordance with the decided frequency andamplitude, a zero oscillation setter 93 b to apply no sine wavehydraulic pressure oscillation, and an oscillation switch 93 c toselectively switch between the hydraulic pressure oscillation and zerooscillation. Note that the oscillation switch 93 c is configured toselect the zero oscillation when the BSC operation flag is 0 (during thenormal control) and select the sine wave hydraulic pressure oscillationwhen the BSC operation flag is 1 (during the belt slip control).

During the belt slip control the secondary hydraulic pressure corrector94 decreases the secondary hydraulic pressure on the basis of the phasedifference between the oscillation component due to the oscillationincluded in the actual secondary hydraulic pressure and that included inthe actual gear ratio. The secondary hydraulic pressure corrector 94includes an actual gear ratio calculator 94 a to calculate an actualgear ratio Ratio from a ratio of the primary rotary rate Npri of theprimary rotation sensor 80 and the secondary rotary rate Nsec of thesecondary rotation sensor 81, a first bandpass filter 94 b to extract anoscillation component from a signal representing the actual secondaryhydraulic pressure Psec obtained with the secondary hydraulic pressuresensor 82, and a second bandpass filter 94 c to extract an oscillationcomponent from the calculated data by the actual gear ratio calculator94 a. It further includes a multiplier 94 d to multiply the oscillationcomponents extracted by both bandpass filters 94 b, 94 c, a low passfilter 94 e to extract phase difference information from themultiplication result, a secondary hydraulic pressure correction amountdeterminer 94 f to determine a secondary hydraulic pressure correctionamount on the basis of the phase difference information from the lowpass filter 94 e, a zero correction amount setter 94 g to set thesecondary hydraulic pressure correction amount to zero, and a correctionamount switch 94 h to selectively switch between the secondary hydraulicpressure correction amount and the zero correction amount. Note that thecorrection amount switch 94 h is configured to select the zerocorrection amount when the BSC operation flag is 0 (during the normalcontrol) and select the secondary hydraulic pressure correction amountwhen the BSC operation flag is 1 (during the belt slip control).

FIG. 5 is a basic flowchart for a switching process between the normalcontrol and the belt slip control (=BSC) over the secondary hydraulicpressure executed by the CVT control unit 8 according to the firstembodiment. In the following the respective steps in FIG. 5 aredescribed.

In step S1 following a startup by turning-on of the key, thedetermination on non-BSC permission in step S2 or normal controlreturning process in step S5, the belt type continuously variabletransmission mechanism 4 is normally controlled, and then the flowproceeds to step S2. During the normal control, the BSC operation flagis set to zero.

In step S2 following the normal control in step S1, a determination ismade on whether or not all of the following BSC permission conditionsare satisfied. At the result being YES (all the BSC permissionconditions satisfied), the flow proceeding to step S3, the belt slipcontrol (BSC) is performed. At the result being NO (any of the BSCpermission conditions unsatisfied), the flow returning to step S1, thenormal control is continued. An example of the BSC permission conditionsis as follows:

-   (1) A change rate of the transmitted torque capacity of the belt    type continuously variable transmission mechanism 4 is stable and    small.

This condition (1) is determined by satisfaction of the following twoconditions, for example.

a. |command torque change rate|<predetermined valueb. |command gear ratio change rate|<predetermined value

Herein, the command gear ratio change rate corresponds to thetransmission speed as a change rate of gear ratio by the belt typecontinuously variable transmission 4. The condition that |command torquechange rate|<predetermined value is satisfied not only by a driving orvehicle condition but also by a forcible limit to vehicle accelerationbased on satisfaction of the acceleration limit permitting condition.

-   (2) The estimated accuracy of the input torque to the primary pulley    42 is within a reliable range.    This condition (2) is for example determined on the basis of the    torque information (estimated engine torque) from the engine control    unit 88, the lockup state of the torque converter 2, the operation    state of a brake pedal, a range position and the like.-   (3) The permitted conditions in the above (1) (2) are continued for    a predetermined length of time.    In step S2 whether or not the above conditions (1), (2), (3) are all    satisfied is determined.

In step S3 following the BSC permission determination in step S2 or theBSC continuation determination in step S4, the belt slip control (FIG. 6to FIG. 8) is performed to reduce an input to the belt 44 of the belttype continuously variable transmission mechanism 4 and maintain thebelt 44 in a so-called micro slip state. Then, the flow proceeds to stepS4. During the belt slip control the operation flag is set to 1.

In step S4 following the belt slip control in step S3, a determinationis made on whether or not all of the following BSC continuationconditions are satisfied. At the result being YES (all the BSCcontinuation conditions satisfied), the flow returning to step S3, thebelt slip control (BSC) is continued. At the result being NO (any of theBSC continuation conditions unsatisfied), the flow proceeds to step S5,and the normal control returning process is performed. An example of theBSC continuation conditions is as follows:

-   (1) A change rate of the transmitted torque capacity of the belt    type continuously variable transmission mechanism 4 is small and    stable.

This condition (1) is determined by satisfaction of the following twoconditions, for example.

a. |command torque change rate|<predetermined valueb. |command gear ratio change rate|<predetermined value

Herein, the condition that |command torque change rate|<predeterminedvalue is satisfied not only by a driving or vehicle condition but alsoby a forcible limitation to vehicle acceleration based on satisfactionof the acceleration limit permitting condition.

-   (2) The estimated accuracy of the input torque to the primary pulley    42 is within a reliable range.

This condition (2) is for example determined on the basis of the torqueinformation (estimated engine torque) from the engine control unit 88,the lockup state of the torque converter 2, the operation state of abrake pedal, a range position and the like. Whether or not the aboveconditions (1), (2) are both satisfied is determined. That is, adifference between the BSC permission conditions and the BSCcontinuation conditions is in that the BSC continuation conditionsexclude the continuation condition (3) of the BSC permission conditions.

In step S5 following a determination that any of the BSC continuationconditions is unsatisfied, the normal control returning process (FIG. 9to FIG. 11) is performed to prevent the belt 4 from slipping when thebelt slip control is returned to the normal control. Upon completion ofthe process, the flow returns to step S1 and shifts to the normalcontrol.

FIG. 6 is a flowchart for the entire belt slip control process executedby the CVT control unit 8 according to the first embodiment. FIG. 7 is aflowchart for the torque limit process of the belt slip control processexecuted by the CVT control unit 8 according to the first embodiment.FIG. 8 is a flowchart for the secondary hydraulic pressure oscillationand correction process of the belt slip control process executed by theCVT control unit 8 according to the first embodiment.

First, as apparent from FIG. 6, during the belt slip control in whichthe BSC permission determination and the BSC continuation determinationare continued, a feedback control inhibition process (step S31) in whichthe command secondary hydraulic pressure is obtained using the actualsecondary hydraulic pressure, a torque limit process (step S32) as apreparation for returning to the normal control, and a secondaryhydraulic pressure oscillation and correction process (step S33) for thebelt slip control are concurrently performed.

In step S31 during the belt slip control in which the BSC permissiondetermination and the BSC continuation determination are continued, thefeedback control under which the command secondary hydraulic pressure isobtained using the actual secondary hydraulic pressure detected by thesecondary hydraulic pressure sensor 82 is inhibited.

That is, the hydraulic pressure feedback control in the normal controlis inhibited during the belt slip control since the actual secondaryhydraulic pressure information contains the oscillation component due tothe oscillation, and switched to a basic secondary hydraulic pressurecontrol using a zero deviation. The hydraulic pressure feedback controlis returned when the belt slip control is shifted to the normal control.

In step S32 during the belt slip control in which the BSC permissiondetermination and the BSC continuation determination are continued, thetorque limit process in FIG. 7 is performed. In step S321 of theflowchart in FIG. 7 a “torque limit request from the belt slip control”is defined to be the driver request torque.

In step S33 during the belt slip control in which the BSC permissiondetermination and the BSC continuation determination are continued, thesecondary hydraulic pressure is oscillated and corrected by feedbackcontrol using phase difference information in FIG. 8. In the followingthe steps of the flowchart in FIG. 8 are described.

In step S331 the command secondary hydraulic pressure is oscillated.That is, the sine wave hydraulic pressure with predetermined amplitudeand predetermined frequency is superimposed on the command secondaryhydraulic pressure. The flow proceeds to step S332.

In step S332 following the oscillation of the command secondaryhydraulic pressure in step S331, the actual secondary hydraulic pressureis detected with the secondary hydraulic pressure sensor 82 to detectthe actual gear ratio by calculation based on information on the rotaryrates from the primary rotation sensor 80 and the secondary rotationsensor 81. The flow proceeds to step S333.

In step S333 following the detection of the actual secondary hydraulicpressure and the actual gear ratio in step S332, the actual secondaryhydraulic pressure and the gear ratio are each subjected to the bandpassfilter process to extract their respective oscillation components (sinewave) and multiply them. Then, the multiplied value is subjected to thelow pass filter process and converted to a value expressed by amplitudeand a phase difference θ (cosine wave) between the oscillation of theactual secondary hydraulic pressure and that of the actual gear ratio.The flow proceeds to step S334. Herein, where A is the amplitude of theactual secondary hydraulic pressure and B is the amplitude of the actualgear ratio, the oscillation of the actual secondary hydraulic pressureis expressed by the formula (1): Asinωt. The oscillation of the actualgear ratio is expressed by the formula (2): Bsin (wt+θ). The formulas(1) and (2) are multiplied, and using the following product sum formula(3):

sinαsinβ=−½{cos(α+β)−cos(α−β)}

the following formula (4):

Asinωt×Bsin(ωt+θ)=(½)ABcosθ−(½)ABcos(2ωt+θ)

is obtained.In the formula (4), (½)ABcos(2ωt+θ) as the double component of theoscillation frequency is reduced through the low pass filter so that theformula (4) becomes the following formula (5):

Asinωt×Bsin(ωt+θ)≈(½)Abcosθ

That is, the multiplied value of the oscillation components included inthe actual secondary hydraulic pressure and the actual gear ratio issubjected to low pass filtering and converted to a value of theamplitudes A, B (constant) multiplied by cosθ (cosine of a phasedifference θ). The converted value can be used for control informationsignifying a phase difference θ in oscillation between the actualsecondary hydraulic pressure and the actual gear ratio (hereinafter,simply phase difference θ).

In step S334 following the calculation of the phase difference θ in theoscillation between the actual secondary hydraulic pressure and theactual gear ratio, a determination is made on whether or not the phasedifference θ is such that 0≦phase difference θ<predetermined value at 1(micro slip range). At the result being YES (0≦phase differenceθ<predetermined value at 1), the flow proceeds to step S335 while at theresult being NO (predetermined value at 1≦phase difference θ), the flowproceeds to step S336.

In step S335 following the determination on 0≦phase differenceθ<predetermined value at 1 (micro slip range) in step S334, thesecondary hydraulic pressure correction amount is set to −Δpsec. Theflow proceeds to step S339.

In step S336 following the determination on the predetermined value at1≦phase difference θ in step S334, a determination is made on whether ornot the phase difference θ is such that predetermined value at 1≦phasedifference θ<predetermined value at 2 (a phase difference range in whichthe belt slip rate falls in a target “micro slip” range). At the resultbeing YES (predetermined value at 1≦phase difference θ<predeterminedvalue at 2), the flow proceeds to step S337 while at the result being NO(predetermined value at 2≦phase difference θ), the flow proceeds to stepS338.

In step S337 following the determination on predetermined value at1≦phase difference θ≦predetermined value at 2 (target slip range) instep S336, the secondary hydraulic pressure correction amount is set tozero and the flow proceeds to step S339.

In step S338 following the determination on predetermined value at2≦phase difference θ (micro/macro slip transition range) in step S336,the secondary hydraulic pressure correction amount is set to +ΔPsec andthe flow proceeds to step S339.

In step S339 following the setting of the secondary hydraulic pressurecorrection amounts in steps S335, S337, S338, the command secondaryhydraulic pressure is set to the value of the basic secondary hydraulicpressure+secondary hydraulic pressure correction amount. Then, the flowends.

FIG. 9 is a flowchart for a returning process from the belt slip controlto the normal control executed by the CVT control unit 8 according tothe first embodiment. FIG. 10 is a flowchart for the torque limitprocess of the returning process to the normal control executed by theCVT control unit 8 according to the first embodiment. FIG. 11 is aflowchart for transmission speed limiting process in the returningprocess to the normal control executed by the CVT control unit 8according to the first embodiment.

First, as apparent from FIG. 9, while the normal control is returnedfrom the belt slip control from the BSC continuation termination to thestart of the normal control, a feedback control returning process (stepS51) in which the command secondary hydraulic pressure is obtained usingthe actual secondary hydraulic pressure, a torque limit process (stepS52) as a preparation for returning to the normal control, anoscillation and correction secondary hydraulic pressure resettingprocess (step S53) for the belt slip control, and a transmissionrestricting process (step S54) in which the transmission speed isrestricted are concurrently performed.

In step S51, while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the feedback control in which the command secondary hydraulicpressure is obtained using the actual secondary hydraulic pressuredetected by the secondary hydraulic pressure sensor 82 is returned.

In step S52 while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the torque limit process as a preparation for returning to thenormal control in FIG. 10 is performed.

In step S53 while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the secondary hydraulic pressure oscillation and correction inFIG. 8 is reset to wait for the normal control.

In step S54 while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the transmission restricting process in which the transmissionspeed is restricted in FIG. 11 is performed.

In the following the steps of the flowchart showing the torque limitprocess in FIG. 10 are described. The key point of this torque limitprocess is to switch the controls on the basis of a magnitude relationamong the three values of driver request torque, torque limit requestfrom the BSC, and torque capacity (calculated torque capacity). Herein,the driver request torque refers to an engine torque requested by adriver, torque limit request from the BSC refers to torque limit amountshown in the phases (2), (3) in FIG. 20. Torque capacity is generally anallowable designed torque capacity and set to a value higher than thedriver request torque by a margin with mechanical variation of the belttype continuously variable transmission mechanism 4 taken intoconsideration, for the purpose of preventing the belt slip. Herein, theactual torque capacity is controlled under the secondary hydraulicpressure control. Further, the calculated torque capacity refers to atorque capacity during the returning process (phase (3) in FIG. 20) ofthe BSC (phase (2) in FIG. 20). The calculated torque capacity isspecifically a value based on or calculated from the actual secondaryhydraulic pressure and the actual gear ratio (torque capacity of one ofthe two pulleys 42, 43 to which engine torque is input, that is, theprimary pulley 42).

In step S521 a determination is made on whether or not the driverrequest torque is larger than the torque limit request from the BSC. Atthe result being YES, the flow proceeds to step S522 while at the resultbeing NO, the flow proceeds to step S525.

In step S522 following the determination on the driver request torque islarger than the torque limit request from the BSC in step S521, adetermination is made on whether or not the calculated torque capacityis larger than the torque limit request from the BSC. At the resultbeing YES, the flow proceeds to step S523 while at the result being NO,the flow proceeds to step S524.

In step S523 following the determination on the calculated torquecapacity>the torque limit request from the BSC in step S522, the torquelimit request from the BSC is set to a smaller one of the torque limitrequest from the BSC (previous value) +ΔT and the calculated allowabletorque capacity. The flow proceeds to RETURN.

In step S524 following the determination on the calculated torquecapacity≦the torque limit request from the BSC in step S522, the torquelimit request from the BSC is set to a smaller one of the torque limitrequest from the BSC (previous value) and the driver request torque. Theflow proceeds to RETURN.

In step S525 following the determination on the driver requesttorque≦the torque limit request from the BSC in step S521, adetermination is made on whether or not the calculated torque capacityis larger than the torque limit request from the BSC. At the resultbeing YES, the flow proceeds to step S527 while at the result being NO,the flow proceeds to step S526.

In step S526 following the determination on the calculated torquecapacity≦the torque limit request from the BSC in step S525, the torquelimit request from the BSC is set to a smaller one of the torque limitrequest from the BSC (previous value) and the driver request torque. Theflow proceeds to RETURN.

In step S527 following the determination on the calculated torquecapacity>the torque limit request from the BSC in step S525, the torquelimit request from the BSC is cancelled. The flow ends.

In the following the steps of the flowchart showing the limiting processfor the transmission speed of a gear ratio to limit a target primaryrotary rate in FIG. 11 are described.

In step S541 a target inertia torque is calculated. The flow proceeds tostep S542.

In step S542 following the calculation of the target inertia torque instep S541, a target primary rotation change rate is calculated from thetarget inertia torque. Then, the flow proceeds to step S543.

In step S543 following the calculation of the target primary rotationchange rate in step S542, a limited target primary rotary rate notexceeding the target primary rotation change rate is calculated, and theflow proceeds to step S544.

In step S544 following the calculation of the limited target primaryrotation change rate in step S543. the transmission control is performedon the basis of the limited target primary rotary rate, and the flowproceeds to step S545.

In step S545 following the transmission control in step S544, adetermination is made on whether or not the transmission control basedon the limited target primary rotary rate is completed or the actualprimary rotary rate has reached the limited target primary rotary rate.At the result being YES (completion of transmission control), the flowends while at the result being NO (in the middle of transmissioncontrol), and the flow returns to step S541.

FIG. 12 is a flowchart for the entire BSC permission determining processexecuted by the CVT control unit 8 according to the first embodiment.FIG. 13 shows a threshold characteristic as a threshold 1 for throttleopening used when an ECO switch is turned on in the BSC permissiondetermining process executed by the CVT control unit 8 according to thefirst embodiment. FIG. 14 shows a threshold characteristic as athreshold 2 for throttle opening used when an ECO switch is turned offin the BSC permission determining process executed by the CVT controlunit 8 according to the first embodiment. In the following the BSCpermission determining process is described with reference to FIG. 12 toFIG. 14.

In step S21 a determination is made on whether or not the ECO switch 89is ON to allow the driver to select the normal drive mode or economicaldrive mode. At the result being YES (economical drive mode selected),the flow proceeds to step S22 and at the result being NO (normal drivemode selected), the flow proceeds to step S26 (limit determining means).The on-state of the ECO switch 89 is one of predetermined accelerationlimit permitting conditions and indicates a driver's selection of theeconomical drive mode, aiming for improving fuel economy.

In step S22 following that the economical drive mode is selected in stepS21, a determination is made on whether or not the throttle openingspeed is a threshold 1 or less. At the result being YES (throttleopening speed≦threshold 1), the flow proceeds to step S23 while at theresult being NO (throttle opening speed>threshold 1), the flow proceedsto step S25 (limit determining means). That throttle openingspeed≦threshold 1 is one of predetermined acceleration limit permittingconditions and indicates that the driver operates the acceleratorwithout aiming for a great acceleration request during the economicaldrive mode. The throttle opening speed is found by differentialoperation of a detected throttle opening from the throttle openingsensor 87 with time. The threshold 1 has characteristics as shown inFIG. 13, remains high while the throttle opening is low and the higherthe throttle opening, the lower the threshold 1, and remains low whilethe vehicle speed is high and the lower the vehicle speed, the lower thethreshold 1. During the economical drive mode, to prioritize fueleconomy to the driver's acceleration request, the range, throttleopening speed ≦threshold 1 is set to be larger than the range ofthreshold characteristic in FIG. 14, throttle opening speed≦threshold 2.In FIG. 13 the higher the throttle opening, the lower the threshold 1because a driver highly demands for the acceleration when pressing downthe accelerator at the high throttle opening. Also, the lower thevehicle speed, the lower the throttle opening 1 because a driver highlydemands for the acceleration when pressing down the accelerator at a lowvehicle speed.

In step S23 following that throttle opening speed≦threshold 1 isdetermined in step S22, the target gear ratio is limited to thepredetermined value and the flow proceeds to step S24 (transmissionspeed limiting means).

Herein, the limitation of the target gear ratio to the predeterminedvalue means the limitation of a command gear ratio change rate so that|command gear ratio change rate|<predetermined value as BSC operationpermitting condition (BSC permission and continuation conditions) issatisfied. That is, the predetermined value in step S23 is the same asthe threshold for the ;command gear ratio change rate in the BSCpermission and continuation conditions in steps S2, S4.

In step S24 following limit or no limit to the target gear ratio changerate in step S23, S25 and S27, BSC permission or BSC continuation isdetermined according to the BSC operation permitting condition. Then,the flow ends.

In step S25 following that throttle opening speed>threshold 1 isdetermined, the target gear ratio change rate is not limited. The flowproceeds to step S24.

In step S26 following that the selection of the normal drive mode isdetermined in step S21, a determination is made on whether or not thethrottle opening speed is threshold 2 or less. At the result being YES(throttle opening speed≦threshold 2), the flow proceeds to step S23. Atthe result being NO (throttle opening speed>threshold 2), the flowproceeds to step S27 (limit determining means). That throttle openingspeed≦threshold 2 is one of the predetermined acceleration limitpermitting conditions and indicates that the driver operates theaccelerator without aiming for a great acceleration request during thenormal drive mode. The throttle opening speed is found by differentialoperation of a detected throttle opening from the throttle openingsensor 87 with time. Similarly to the threshold 1, the threshold 2 hascharacteristics as shown in FIG. 14, remains high while the throttleopening is low and the higher the throttle opening, the lower thethreshold 2, and remains high while the vehicle speed is high and thelower the vehicle speed, the lower the threshold 2. During the normaldrive mode, to prioritize the driver's acceleration request to fueleconomy, the range, throttle opening speed≦threshold 2 is set to besmaller than the range of threshold characteristic in FIG. 13, throttleopening speed≦threshold 1.

In step S27 following that throttle opening speed>threshold 2 isdetermined in step S26, the target gear ratio change rate is notlimited. The flow proceeds to step S24.

Next, the control and operation of the belt type continuously variabletransmission mechanism 4 according to the first embodiment is described.It will be divided into six parts, normal control and belt slip control,BSC permission and continuation determining operations, BSC permissionand continuation determining operations by command gear ratio changerate |<predetermined value, operation range expanding operation in whichthe belt slip control is permitted, belt slip control (BSC operation),and returning control operation from the BSC to the normal control.

[Normal Control and Belt Slip Control]

The belt type continuously variable transmission 4 according to thefirst embodiment is configured to control primary and secondaryhydraulic pressures. The normal control refers to control over the belt44 wound around the pulleys 42, 43 not to slip while the belt slipcontrol refers to control over the belt 44 to intentionally slip at apredetermined slip rate. In the following the essential terms, normalcontrol and belt slip control and the reason for adopting a phasedifference feedback control are described.

In the normal control the primary and secondary hydraulic pressures arecontrolled to generate belt clamp force (=belt thrust) sufficient tocertainly prevent the belt 44 from slipping even with a fluctuation ininput torque from the engine 1 as a driving source. During the normalcontrol the actual secondary hydraulic pressure from the secondaryhydraulic pressure sensor 82 is controlled by hydraulic pressurefeedback control (PI control) to be a target hydraulic pressurecalculated in the basic hydraulic pressure calculator 90 on the basis ofinput torque or required thrust difference at transmission (FIG. 4).

Meanwhile, in the belt slip control the secondary hydraulic pressure iscontrolled to maintain the so-called micro-slip of the belt 44 bylowering the belt clamp force from that in the normal control under thesame driving condition. In the belt slip control the secondary hydraulicpressure is oscillated to extract an oscillation component included inthe actual secondary hydraulic pressure and that included in the actualgear ratio due to the oscillation and adjust a phase difference θ in theextracted oscillation components to fall within a target range(predetermined value 1≦phase difference<predetermined value 2) by phasedifference feedback control (FIG. 8).

The reason for adopting the phase difference feedback control in thebelt slip control is that while no belt slip is occurring with no changein the contact positions of the secondary pulley 43 and the belt 44, theextracted oscillation components in the actual secondary hydraulicpressure and the actual gear ratio are in synchronous wave forms at thesame phase. However, when a bet slip occurs due to a change in thecontact positions of the secondary pulley 43 and the belt 44, the phasedifference in the oscillation waveforms becomes larger in proportion toan increase in the belt slip rate. In other words the phase differenceand the belt slip rate are closely correlated with each other so thatusing phase difference information to estimate the belt slip rate, thebelt slip control is feasible with high accuracy to slip the belt 44 inthe micro slip range without directly detecting the belt slip rate.

In addition, the phase difference information is acquired from actualgear ratio information from the primary and secondary rotation sensors80, 81 and actual secondary hydraulic pressure information from thesecondary hydraulic pressure sensor 82. This eliminates the necessityfor adding a new sensor for acquiring slip rate information in the beltslip control. The belt slip control is performed using the existingsensors 80, 81, 82 for the normal control of the belt type continuouslyvariable transmission 4.

However, if the basic component of the actual secondary hydraulicpressure is found using a deviation calculated from the actual secondaryhydraulic pressure information containing the oscillation component fromthe secondary hydraulic pressure sensor 82, the secondary hydraulicpressure cannot be stably controlled due to a fluctuation in thecalculated deviation caused by the oscillation. Because of this, in thebelt slip control the basic component of the actual secondary hydraulicpressure is found by zero deviation.

[BSC Permission and Continuation Determining Operations]

At a start of the vehicle's running, the flow proceeds to step S2 fromstep S1 in the flowchart in FIG. 5. Unless all the BSC permissiondetermining conditions are satisfied in step S2, the flow from step S1to step S2 is repeated to continue the normal control. That is, thesatisfaction of all the BSC permission determining conditions in step S2is defined to be BSC control starting condition.

The BSC permission conditions in the first embodiment are as follows:

(1) A change rate of the transmitted torque capacity of the belt typecontinuously variable transmission mechanism 4 is stable and small.

(2) The estimated accuracy of the input torque to the primary pulley 42is within a reliable range.

(3) The permitted conditions in the above (1) (2) are continued for apredetermined length of time. In step S2 whether or not the aboveconditions (1), (2), (3) are all satisfied is determined.

Thus, the belt slip control is allowed to start if a change rate of thetransmitted torque capacity of the belt type continuously variabletransmission mechanism 4 continues to be stably small and the estimatedaccuracy of the input torque to the primary pulley 42 is continuouslywithin a reliable range for a predetermined length of time during thenormal control. As above, the belt slip control is permitted to startupon the satisfaction of all the BSC permission conditions so that it isable to start the belt slip control in a preferable vehicle drivingcondition with an assured high control precision.

After the BSC permission is determined in step S2, in step S3 the beltslip control is performed to reduce an input to the belt 44 of the belttype continuously variable transmission mechanism 4 and maintain thebelt 44 in a target micro-slip state. Then, in step S4 following thebelt slip control in step S3, a determination is made on whether or notall of the BSC continuation conditions are satisfied. As long as all ofthe BSC continuation conditions are satisfied, the flow from step S3 tostep S4 is repeated to continue the belt slip control (BSC).

Here, the BSC continuation conditions are the BSC permission conditions(1), (2) and exclude the continuation condition for a predeterminedlength of time (3) of the BSC permission conditions. This is because thebelt slip control is immediately stopped and returned to the normalcontrol if one of the conditions (1), (2) is unsatisfied during the beltslip control. Accordingly, it is made possible to prevent the belt slipcontrol from continuing in a vehicle driving condition with uncertaincontrol precision.

[BSC Permission and Continuation Determining Operations by |Command GearRatio Change Rate|<Predetermined Value]

In the belt slip control permission determination according to the firstembodiment, the belt slip control is permitted under one of theconditions (1) that a command gear ratio change rate indicating a changerate of the gear ratio of the belt type continuously variabletransmission 4 is less than the predetermined value.

Specifically, at a small transmission change rate (change range of gearratio per unit time=transmission speed), the oscillation componentoccurs due to oscillation during transmission as shown in an actual gearratio characteristic relative to a target gear ratio characteristic inFIG. 15 but it can be separated from the oscillation component due to achange in gear ratio. Thus, the belt slip condition can be estimatedwith high accuracy by monitoring a phase difference in the oscillationcomponents of the actual gear ratio.

Meanwhile, at a large transmission change rate, the oscillationcomponent included in the actual gear ratio disappears as shown in thearea C in FIG. 16 so it cannot be separated from the oscillationcomponent due to the gear ratio change. Thus, the belt slip conditioncannot be estimated with accuracy by monitoring a phase difference inthe oscillation components of the actual gear ratio.

According to the first embodiment, however, the belt slip control ispermitted when |command gear ratio change rate|<predetermined value andthe belt slip condition is estimated with high accuracy. Because ofthis, a reduction in the secondary hydraulic pressure reduces a beltfriction, thereby lowering the driving load of the transmission.Accordingly, the in-use fuel economy of the engine 1 can be improved.

Meanwhile, the belt slip control is not permitted when |command gearratio change rate|≦predetermined value and the belt slip condition isnot estimated with accuracy. Therefore, it is possible to prevent thebelt from being placed in a macro slip state or largely slipped duringthe belt slip control, which would otherwise occur when the belt slipcontrol is permitted with transmission speed condition not taken intoaccount, for example. That is, the belt micro-slip state is maintainedby reducing the secondary hydraulic pressure and the belt clamp forceduring the belt slip control from those during the normal control. Anincrease in the input torque to the belt type continuously variabletransmission 4 in this state may cause the belt 44 supported with lowclamp force to be greatly slipped as macro slip.

Relative to the BSC permission condition, |command gear ratio changerate|<predetermined value, the predetermined value as a threshold fordetermining the magnitude of the command gear ratio change rate is setto a value which allows the extraction of the oscillation componentincluded in the actual gear ratio Ratio. For example, the predeterminedvalue is set to a value obtained by subtracting a margin of variation inproduct quality from an upper limit gear ratio change rate which isdetermined to be a limit to be able to extract the oscillation componentin the actual gear ratio Ratio while gradually increasing thetransmission speed of the belt type continuously variable transmission4.

The belt slip control system is configured that the sine waveoscillation controller 93 in FIG. 4 superimposes a sine wave hydraulicpressure oscillation to the command secondary hydraulic pressure foroscillation, to estimate a belt slip condition using the oscillationcomponents due to the oscillation included in the actual secondaryhydraulic pressure and in the gear ratio Ratio. In other words,extracting the oscillation component due to the oscillation from theactual gear ratio Ratio is an essential condition for the execution ofthe belt slip control with the estimated accuracy of the belt slipcondition maintained.

Thus, under the belt slip control permission determining condition that|command gear ratio change rate|<predetermined value, it is possible tomaintain the estimated accuracy of the belt slip condition on the basisof the two extracted oscillation components. In addition, it is madepossible to expand the operation range under which the belt slip controlpermitting condition on the gear ratio change rate is satisfied, byallowing an increase in the gear ratio change rate to be in a limitrange to be able to extract the oscillation component in the actual gearratio Ratio, compared with the belt slip control permitted only under acondition that the gear ratio change rate is constant.

The first embodiment is configured that the belt slip control ispermitted not on the basis of the actual gear ratio change rate of thebelt type continuously variable transmission 4 but when the command gearratio change rate as a control command is less than the predeterminedvalue. Therefore, the belt slip control start and continuation aredetermined at the time when a target gear ratio is found by operation tocalculate the command gear ratio change rate from a current gear ratioand the target gear ratio. Accordingly, the belt slip control start andcontinuation can be determined on the basis of prognostic information asthe command gear ratio change rate before the gear ratio of the belttype continuously variable transmission 4 is actually changed.

[Operation Range Expanding Operation to Permit Belt Slip Control]

During vehicle running with the ECO switch 89's ON and the throttleopening speed at the threshold 1 or less, the flow in FIG. 12 proceedsto step S21, S22, S23 to S24. That is, economical drive mode selectingcondition as the turning-on of the ECO switch 89 and throttle openingspeed condition as throttle opening speed≦threshold 1 are both satisfied(YES in steps S21, S22) and a limit to acceleration of the vehicle ispermitted under the acceleration limit permitting condition. Followingthis permission, in step S23 the target gear ratio change rate islimited to the predetermined value so as to satisfy |command gear ratiochange rate|<predetermined value and in step S24 the BSC permission andcontinuation to start and continue the belt slip control are determinedin accordance with the BSC operation permitting condition.

During vehicle running with the ECO switch 89's ON and the throttleopening speed exceeding the threshold 1, the flow in FIG. 12 proceeds tostep S21, S22, S25 to S24. That is, the economical drive mode selectingcondition as the turning-on of the ECO switch 89 is satisfied (YES instep S21) but throttle opening speed condition as throttle openingspeed>threshold 1 is not (No in step S22), so that vehicle accelerationbased on an acceleration request is permitted. Following thispermission, in step S25 the target gear ratio change rate is not limitedand in step S24 basically, maintaining normal control and returning tonormal control are determined in accordance with the BSC operationpermitting condition.

During vehicle running with the ECO switch 89's OFF and the throttleopening speed equal to or below the threshold 2, the flow in FIG. 12proceeds to step S21, S26, S23 to S24. That is, even in the normal drivemode with the ECO switch 89 turned off, upon satisfaction of thecondition that throttle opening speed≦threshold 2 (YES in step S26), thelimit to vehicle acceleration is permitted. In step S23, following thispermission, the target gear ratio change rate is limited to thepredetermined value to satisfy |command gear ratio changerate|<predetermined value. In step S24 the BSC permission andcontinuation to start and continue the belt slip control are determinedin accordance with the BSC operation permitting condition.

During vehicle running with the ECO switch 89's OFF and the throttleopening speed equal exceeding the threshold 2, the flow in FIG. 12proceeds to step S21, S26, S27 to S24. That is, the normal drive modeselecting condition is satisfied by the turning-off of the ECO switch 89(YES in step S21) but the throttle opening speed condition as throttleopening speed >threshold 2 (NO in step S26) is not, so that vehicleacceleration based on an acceleration request is permitted. Followingthe permission, the target gear ratio change rate is not limited in stepS27 and in step S24 basically, maintaining normal control and returningto normal control are determined in accordance with the BSC operationpermitting condition.

As described above, in the belt slip control permission determinationaccording to the first embodiment, one of the conditions for permittingthe belt slip control is that the command gear ratio change rateindicating the transmission speed of the belt type continuously variabletransmission 4 is less than the predetermined value. Moreover, the beltslip control is actively performed by forcibly limiting the target gearratio change rate when the limit to the vehicle acceleration isdetermined as permissive according to the acceleration limit permittingcondition.

FIG. 17 is a timing chart for the characteristics of limitdetermination, target gear ratio, BSC operation flag (comparison),limited target gear ratio, and limited BSC operation flag. FIG. 18 is agraph showing the effects of an expanded BSC operation range accordingto the first embodiment in comparison with a case where the targettransmission change rate (target transmission speed) is not activelylimited. In the following the operation range expanding operation topermit the belt slip control is described with FIGS. 17, 18.

First, the belt slip control aims to enhance the power transmissionefficiency of the belt type continuously variable transmission 4 byreducing belt friction and friction loss, and improve the fuel economyof an engine vehicle or a hybrid vehicle. To improve fuel economy, it isimportant for realizing an effective belt slip control to expand theoperation range in which the belt slip control is performed as much aspossible.

For comparison, an example is shown where the only belt slip controlpermitting condition is such that the command gear ratio change rate isset according to a vehicle condition (accelerator opening, vehiclespeed, and else) to be equal to or less than a predetermined value orless. While the target gear ratio is being fluctuated by a driver'sslight change to the accelerator opening or else as shown in thecharacteristic T in FIG. 17, the BSC operation flag rises in smalldivided operation ranges at time t2 to t3, t4 to t5, t6 to t7, t8 to t9,t10 to t11, and after t13 and the belt slip control is performed inthese periods. That is, in this example the operation range in which thecondition that |command gear ratio change rate|≦predetermined value issatisfied is limited since whether or not to permit the belt slipcontrol is passively determined depending on a driver's acceleratormanipulation or a vehicle condition as vehicle speed. Especially, if adriver tends to slightly press down and release the acceleratorrepeatedly during constant-speed running, the operation range in whichthe belt slip control is performed will be very limited.

Meanwhile, in the first embodiment the target gear ratio change rate isforcibly limited upon satisfaction of the acceleration limit permittingcondition. Thereby, the belt slip control is actively performed notdepending on a driver's accelerator manipulation or a vehicle conditionbut upon satisfaction of the condition that |command gear ratio changerate|≦predetermined value.

Specifically, while the target gear ratio is being fluctuated by adriver's slight change to the accelerator opening or a slight change invehicle speed on a slope as shown in the characteristic T in FIG. 17, attime t1 the target gear ratio change rate (target transmission speed) islimited to the predetermined value to satisfy |command gear ratio changerate|<predetermined value upon satisfaction of both of the accelerationlimit permitting condition as the selection of the economical drivingmode and the throttle opening speed condition as throttle openingspeed≦threshold 1. Then, the BSC operation flag is set to start the beltslip control. While the acceleration limit permitting condition andthrottle opening speed condition are both satisfied, the target gearratio change rate is continuously limited with the BSC operation flagkept set. When at time t12 the throttle opening speed becomes largerthan the threshold 1 and the throttle opening speed condition isunsatisfied, the limitation to the target gear ratio change rate isstopped with the BSC operation flag set off to end the belt slipcontrol. As shown in the limited target gear ratio characteristic T′ inFIG. 17, the target gear ratio change rate (=transmission speed) iscontinuously limited to maintain the belt slip control in the period Gfrom the point FS at time t1 to the point FE at time t12.

Compared with this example, the period in which the belt slip control isperformed according to the first embodiment is a sum of that in theexample and the periods t1 to t2, t3 to t4, t5 to t6, t7 to t8, t9 tot10, and t11 to t12 during which |command gear ratio changerate|≦predetermined is unsatisfied.

That is, in FIG. 18 the BSC operation range is expanded as indicated bythe arrow W by adding a BSC operation range BE2 in which the vehicleacceleration limit permitting condition is satisfied to a BSC operationrange BE1 in the compared example. Further, the estimated accuracy ofthe belt slip condition can be maintained by limiting the target gearratio change rate to less than a BSC operation limit gear ratio changerate (=predetermined value) in the added BSC operation range BE2 fromthe target gear ratio change rate decided by an acceleration request anda vehicle performance, as indicated by the arrow L in FIG. 18.

Thus, in the driving condition such as the ECO switch 89's turning-on inwhich a limitation to vehicle acceleration is permitted, it is possibleto improve fuel efficiency by forcibly limiting the gear ratio changerate to a BSC operable gear ratio change rate to actively perform thebelt slip control and expand the BSC operation range.

[Belt Slip Control Operation (BSC operation)]

At start of the belt slip control, the secondary hydraulic pressure isset to a value to acquire the clamp force not to cause belt slippagewith estimated safety so that the condition that the phase difference θis lower than the predetermined value 1 is satisfied. In the flowchartin FIG. 8 the flow from step S331→step S332→step S333→step S334→stepS335 to step S339 is repeated and every time the flow is repeated, thecommand secondary hydraulic pressure is decreased in response to thecorrection by −Δpsec. Then, until the phase difference θ at 1 or morereaches the predetermined value at 2, the flow proceeds from stepS331→step S332→step S333→step S334→step S336→step S337 to step S339 inFIG. 8 to maintain the command secondary hydraulic pressure. At thephase difference θ being the predetermined value at 2 or more, the flowproceeds from step S331→step S332→step S333→step S334→step S336→stepS338 to step S339 to increase the command secondary hydraulic pressurein response to the correction by +Δpsec. Under the belt slip control theslip rate is maintained to be in a micro slip range by phase differencefeedback control so that the phase difference θ falls within the rangeof the predetermined value 1 or more to less than the predeterminedvalue 2.

The belt slip control is described with reference to the timing chart inFIG. 19. At time t1, the above BSC permission conditions (1), (2) aresatisfied and continued (BSC permission condition (3)). From time t2 totime t, at least one of the above BSC continuation conditions (1), (2)becomes unsatisfied, and the BSC operation flag and SEC pressure F/Binhibiting flag (secondary pressure feedback inhibiting flag) are setfor the belt slip control. A little before time t3 the accelerator ispressed, so that at least one of the BSC continuation conditions becomesunsatisfied and the control to return to the normal control is performedfrom time t3 to time t4. After time t4, the normal control is performed.

Thus, as apparent from the accelerator opening characteristic, vehiclespeed characteristic, and engine torque characteristic as well as thesolenoid current correction amount characteristic of the secondaryhydraulic pressure solenoid 75 during steady running determinationindicated by the arrow E in FIG. 19, under the belt slip control thephase difference θ between the oscillation components of the secondaryhydraulic pressure due to the oscillation and the gear ratio ismonitored to increase or decrease the current value. Note that thesecondary hydraulic pressure solenoid 75 is normally open (always open)and decreases the secondary hydraulic pressure along with a rise of thecurrent value.

The actual gear ratio is maintained to be virtually constant by the beltslip control although it fluctuates with small amplitude as shown in theactual gear ratio characteristic (Ratio) in FIG. 19. The phasedifference θ, as shown in the phase difference characteristics of theSEC pressure oscillation and Ratio oscillation in FIG. 19, graduallyincreases with time from time t2 when the slip rate is approximatelyzero, and reaches a target value (target slip rate). The secondaryhydraulic pressure as shown in the SEC hydraulic pressure characteristicin FIG. 19 decreases with time from time t2 when safety is secured, asindicated by the arrow F, and reaches a value of the designed minimumpressure added with hydraulic pressure amplitude in the end which is inthe hydraulic pressure condition with a margin to the actual minimalpressure. While the belt slip control continues for a long time, theactual secondary hydraulic pressure is maintained in the amplitude rangeof the designed minimum pressure plus hydraulic pressure to maintain thetarget value of the phase difference θ (of slip rate).

Thus, a decrease in the secondary hydraulic pressure by the belt slipcontrol results in reducing the belt friction acting on the belt 44 andreducing the drive load on the belt type continuously variabletransmission mechanism 4 by the reduction in the belt friction. As aresult, it is possible to improve the in-use fuel economy of the engineI without affecting the travelling performance during the belt slipcontrol based on the BSC permission determination.

[Returning Control from BSC to Normal Control]

During the belt slip control while the BSC permission and continuationdeterminations are continued, the torque limit process in step S32 inFIG. 6 is performed by setting the “torque limit request from the beltslip control” as the driver request toque in step S321 in FIG. 7. In thefollowing torque limit operation for retuning to the normal control isdescribed with reference to FIG. 10 and FIG. 20.

The engine control unit 88 has a limit torque amount as an upper controllimit engine torque, and controls the actual torque of the engine 1 notto exceed the limit torque amount. This limit torque amount isdetermined according to various requests. For example, the upper limitinput torque to the belt type continuously variable transmissionmechanism 4 is set to the torque limit request during the normal control(phase (1) in FIG. 20), and the CVT control unit 8 sends the torquelimit request during the normal control to the engine control unit 88.The engine control unit 88 selects the minimum one of torque limitrequests from various controllers as the limit torque amount.

Specifically, at time t5 the phase (1) of the normal control is shiftedinto the belt slip control, and the torque limit request from the BSC issent to the engine control unit 88 in the phase (2) as shown in thelimit torque amount characteristic in FIG. 20. However, the torque limitrequest from the BSC during the BSC (phase (2) in FIG. 20) is forpreparation in advance for the torque limiting in FIG. 10 and does notvirtually function as a torque limit during the BSC (phase (2) in FIG.20).

Then, at time t6 the BSC continuation is aborted and shifted into thecontrol to return to the normal control. At time t6 a torque limitrequest is issued because of the driver request torque>torque limitrequest from the BSC and the calculated torque capacity≦torque limitrequest from the BSC. Therefore, the flow from step S521→step S522→stepS524 to RETURN in the flowchart in FIG. 10 is repeated to maintain thetorque limit request from the BSC (previous value) in step S524.

Thereafter, at time t7 the driver request torque>torque limit requestfrom the BSC and the calculated torque capacity>torque limit requestfrom the BSC. The flow from step S521→step S522→step S523 to RETURN inFIG. 10 is repeated to gradually increase the torque limit request fromthe BSC to be (previous value+ΔT). Along with this rising gradient, theactual torque gradually rises.

Due to the rise of the torque limit request from the BSC since time t7,at time t8 the driver request torque≦torque limit request from the BSCand the calculated torque capacity>torque limit request from the BSC.The flow proceeds from step S521→step S525→step S527 to END in theflowchart in FIG. 10. In step S527 the torque limit from the BSC iscancelled.

In this example the flow skips step S526 which is executed when theaccelerator is manipulated as stepped on or returned (released) for ashort period of time. Specifically, step S526 is skipped when the beltslip control is cancelled by stepping-on of the accelerator and theaccelerator is released as soon as the returning control starts.

In returning from the belt slip control to the normal control, bylimiting the change speed of the input torque to the belt typecontinuously variable transmission 4, it is made possible to prevent anexcessive increase in the input torque relative to the belt clamp forceand prevent the belt from greatly slipping due to a sudden increase inthe belt slip rate from micro slip to macro slip.

In returning from the belt slip control to the normal control, with achange in the gear ratio at a normal transmission speed while the changespeed of the input torque to the belt type continuously variabletransmission mechanism 4 is suppressed by the above torque limitcontrol, the input torque is significantly reduced according to a changein the rotary inertia. This makes a driver feel unnecessary impact(deceleration). In view of this, the change speed of the gear ratio islimited along with a limit to the change speed of the input torque.

That is, upon the termination of the BSC continuation and shift to thecontrol to return to the normal control, the flow from step S541→stepS542→step S543→step S544 to step S545 in the flowchart in FIG. 11 isrepeated until completion of the transmission. The transmission controlis conducted on the basis of the limited target primary rotation rate.

As described above, limiting the primary rotation change rate orlowering the transmission speed makes it possible to reduce a change inthe rotary inertia and prevent a reduction in the input torque to thetransmission. As a result, it is possible to prevent a driver fromfeeling unnecessary impact (deceleration) in returning to the normalcontrol.

Next, the effects of the control device and method for the belt typecontinuously variable transmission mechanism 4 according to the firstembodiment are described in the following.

-   (1) A control device is for vehicle belt type continuously variable    transmission which includes a primary pulley 42 and a secondary    pulley 43 around which a belt 44 is wound, to control a gear ratio    by controlling a primary hydraulic pressure and a secondary    hydraulic pressure. It includes a belt slip control means (FIG. 8)    configured to oscillate the secondary hydraulic pressure and extract    an oscillation component due to the oscillation included in an    actual gear ratio from a basic component of the actual gear ratio    when a transmission speed is less than a predetermined value, so as    to control the secondary hydraulic pressure on the basis of a phase    difference between the oscillation component due to the oscillation    included in the actual gear ratio and an oscillation component due    to the oscillation included in an actual secondary hydraulic    pressure, the transmission speed being a change speed of the gear    ratio, a limit determining means (steps S21, S22, and S26 in    FIG. 12) configured to determine whether to limit acceleration of    the vehicle on the basis of a predetermined acceleration limit    permitting condition, and a transmission speed limiting means (step    S23 in FIG. 12) configured to limit the transmission speed to less    than the predetermined value when the limit determining means    determines to limit the acceleration of the vehicle.

Thus, it is possible to provide a control device for a vehicle belt typecontinuously variable transmission which can expand the operation rangein which the belt slip control is permitted with the estimated accuracyof a belt slip condition maintained, to thereby improve the reducingeffects of driving energy consumption by a decrease in belt friction.

-   (2) The limit determining means is configured to determine to limit    the acceleration of the vehicle when an economical drive mode is    selected as the predetermined acceleration limit permitting    condition from a normal drive mode and the economical drive mode    (step S21 in FIG. 12).

Thus, in addition to the effects in the item (1), it is possible toexpand the operation range in which the belt slip control is permitted,in response to a driver's intention for having selected the economicaldrive mode.

-   (3) The device further includes a switch (ECO switch 89) configured    to allow a driver to select the normal drive mode and the economical    drive mode. This makes it possible to promptly, certainly reflect,    in the belt slip control, a driver's intention for preferring the    economical drive mode by a simple switch manipulation, in addition    to the effects in the item (2).-   (4) The limit determining means is configured to determine to limit    the acceleration of the vehicle on the basis of the acceleration    limit permitting condition when an increase speed of either an    accelerator opening or a throttle opening is equal to or lower than    a predetermined speed (steps S22 and S26 in FIG. 12, FIGS. 13, 14).

In addition to the effects in the items (1) to (3), it is possible toset the operation range with a low acceleration request level to be theoperation range in which the belt slip control is permitted, inaccordance with a driver's acceleration request represented by anincrease speed of the accelerator opening or throttle opening.

In combination with the selection of driving mode, for example, when adriver has selected the economical drive mode, the operation range inwhich the belt slip control is permitted can be set to an expanded rangewith a high acceleration request level, with fuel economy preferentiallytaken into consideration (FIG. 13). Further, when the driver's selectedmode is the normal drive mode, the operation range in which the beltslip control is permitted can be set to a range with a low accelerationrequest level during normal driving, with acceleration performancepreferentially taken into consideration (FIG. 14).

-   (5) A control method for a vehicle belt type continuously variable    transmission which comprises a pair of pulleys 42, 43 around which a    belt 44 is wound to control a gear ratio by controlling hydraulic    pressures of the pulleys 42, 43, the method comprises the steps of,    when a transmission speed is less than a predetermined value,    oscillating the hydraulic pressure and extracting an oscillation    component due to the oscillation included in an actual gear ratio    from a basic component of the actual gear ratio so as to permit a    belt slip control over the hydraulic pressures on the basis of a    phase difference between the oscillation component included due to    the oscillation in the actual gear ratio and the oscillation    component due to the oscillation included in an actual hydraulic    pressure, the transmission speed being a change rate of the gear    ratio: and limiting the transmission speed to less than the    predetermined value when determining to limit acceleration of the    vehicle on the basis of a predetermined acceleration limit    permitting condition.

Thus, it is possible to provide a control method for a vehicle belt typecontinuously variable transmission which can expand the operation rangein which the belt slip control is permitted with the estimated accuracyof a belt slip condition maintained, to thereby improve the reducingeffects of driving energy consumption by a decrease in belt friction.

Although the control device and method for a vehicle belt typecontinuously variable transmission have been described in terms of theexemplary first embodiment, it is not limited thereto. It should beappreciated that variations or modifications may be made in theembodiments described by persons skilled in the art without departingfrom the scope of the present invention as defined by the followingclaims.

According to the first embodiment, the predetermined value to which thetransmission speed limiting means limits the gear ratio change rate isset to the same value as the threshold for the BSC permission andcontinuation conditions of |command gear ratio change rate|. However, itshould not be limited thereto. Alternatively, it can be set to a smallervalue than the threshold.

The first embodiment has described an example where the normal andeconomical (ECO) drive modes are determined from ON/OFF of the ECOswitch 89 (step S21 in FIG. 12). However, it should not be limitedthereto. Alternatively, the normal drive mode or economical drive modecan be determined, irrespective of the driver' switch manipulation, forexample, by monitoring the driving condition (accelerator or brakeoperation) or by a result of drive mode determination or switching in anautomatic drive mode switch system.

The first embodiment has described an example where a limit to vehicleacceleration is permitted even during the normal drive mode uponsatisfaction of the acceleration limit permitting condition that thethrottle opening speed is equal to or below the threshold 2 (step S26 inFIG. 12). This intends for improvement in fuel economy during the normaldrive mode by expanding the operation range in which the belt slipcontrol is performed. However, the present invention should not belimited to such an example. Alternatively, the limit to vehicleacceleration can be inhibited during the normal drive mode irrespectiveof the throttle opening speed. Accordingly, in the flowchart in FIG. 12,the flow omits step S26 and proceeds to step S27 when the normal drivemode is determined in step S21 (when result is NO) and to step S24 withno limit to the target gear ratio change rate.

The first embodiment has described an example where a limit to vehicleacceleration is permitted upon satisfaction of the acceleration limitpermitting condition that the throttle opening speed is equal to orbelow the threshold 1 during the economical drive mode and that it isequal to or below the threshold 1 during the normal drive mode. Insteadof the throttle opening speed as a throttle value opening speed, a limitto vehicle acceleration can be determined by the accelerator openingspeed as an accelerator pedal pressing speed.

The first embodiment has described an example where a hydraulic pressurecircuit of a single side adjusting type controlled by a step motor isused for the transmission hydraulic pressure control unit 7. However,another single side adjusting type or both sides adjusting typetransmission hydraulic pressure control unit can be also used.

The first embodiment has described an example where only the secondaryhydraulic pressure is oscillated. However, for example, the primaryhydraulic pressure together with the secondary hydraulic pressure can beconcurrently oscillated at the same phase by a direct acting controlsystem. Alternatively, the primary hydraulic pressure together with thesecondary hydraulic pressure can be oscillated at the same phase byoscillating the line pressure.

The first embodiment has described an example of an oscillation meanswhere a signal of oscillation component is superimposed on the commandsecondary hydraulic pressure signal during arithmetic operation.Alternatively, an output signal of oscillation component can besuperimposed on an output solenoid current value.

The first embodiment has described an application example of an enginevehicle incorporating a belt type continuously variable transmission.The present invention is also applicable to a hybrid vehicleincorporating a belt type continuously variable transmission, anelectric vehicle incorporating a belt type continuously variabletransmission and the like. In short it is applicable to any vehicleincorporating a belt type continuously variable transmission whichperforms a hydraulic pressure transmission control.

1. A control device for a vehicle belt type continuously variabletransmission which comprises a primary pulley and a secondary pulleyaround which a belt is wound, to control a gear ratio by controlling aprimary hydraulic pressure and a secondary hydraulic pressure, thedevice comprising: a belt slip control means configured to oscillate thesecondary hydraulic pressure and extract an oscillation component due tothe oscillation from a basic component of an actual gear ratio when atransmission speed is less than a predetermined value, to control thesecondary hydraulic pressure on the basis of a phase difference betweenthe oscillation component of the actual gear ratio and an oscillationcomponent of an actual secondary hydraulic pressure due to theoscillation, the transmission speed being a change speed of the gearratio; a limit determining means configured to determine whether tolimit acceleration of the vehicle; and a transmission speed limitingmeans configured to limit the transmission speed to less than thepredetermined value when the limit determining means determines to limitthe acceleration of the vehicle, wherein the belt slip control means isconfigured to permit the belt slip control while the transmission speedlimiting means is limiting the transmission speed to the predeterminedvalue.
 2. A control device for a vehicle belt type continuously variabletransmission according to claim 1, wherein the limit determining meansis configured to determine to limit the acceleration of the vehicle whenan economical drive mode is selected from a normal drive mode and theeconomical drive mode.
 3. A control device for a vehicle belt typecontinuously variable transmission according to claim 2, furthercomprising a switch configured to allow a driver to select the normaldrive mode or the economical drive mode.
 4. A control device for avehicle belt type continuously variable transmission according to claim1, wherein the limit determining means is configured to determine tolimit the acceleration of the vehicle when an increase speed of eitheran accelerator opening or a throttle opening is equal to or lower than apredetermined speed.
 5. A control method for a vehicle belt typecontinuously variable transmission which comprises a pair of pulleysaround which a belt is wound to control a gear ratio by controlling ahydraulic pressure of the pulleys, the method comprising the steps ofwhen a transmission speed is less than a predetermined value,oscillating the hydraulic pressures and extracting an oscillationcomponent due to the oscillation from a basic component of an actualgear ratio so as to permit a belt slip control over the hydraulicpressure on the basis of a phase difference between the oscillationcomponent of the actual gear ratio and an oscillation component of anactual secondary hydraulic pressure due to the oscillation, thetransmission speed being a change rate of the gear ratio; and limitingthe transmission speed to less than the predetermined value andpermitting the belt slip control when determining to limit accelerationof the vehicle.